Circuit boards that carry electronic circuits as well as discrete electronic components are well known. A circuit board substrate is prepared with predetermined conductor paths and pads for receiving the lead of an electronic component such as integrated circuit chips, resistors or capacitors. During the circuit board fabrication process, solder paste bricks are placed onto the board substrate at appropriate positions. The solder paste is usually applied by placing a screen onto the substrate, applying solder paste through the screen openings and removing the screen from the substrate. The circuit board electronic components are then positioned onto the substrate, preferably with a pick and place machine, with the leads of the electronic components placed on the respective solder paste bricks. The circuit board is passed through an oven after all of the components are positioned on a substrate to melt the solder paste thus creating an electrical as well as mechanical connection between the components and the substrate.
The size of the solder paste bricks and the accuracy with which they must be placed on the substrate has become increasingly smaller and tighter with the increased emphasis on miniaturization in the electronics industry. Solder paste brick heights can be as small as 100 microns and the height of the solder paste brick must often be measured to within 1 percent of the designed height and size. The center-to-center spacing between solder bricks is sometimes 200 microns. Too little solder paste can result in no electrical connection between the lead of an electronic component and the pad of the circuit board substrate. Too much paste can result in bridging and short-circuiting between the leads of a component.
A single circuit board can cost thousands or even tens of thousands of dollars to manufacture. Testing of a circuit board after the fabrication process is complete can detect errors in solder paste placement and component lead connection, but often the only remedy for a faulty board is the rejection of the entire board. It is accordingly imperative that a circuit board be inspected during the fabrication process so that improper solder paste placement can be detected prior to the placement of the electronic components onto the substrate. Such in-process solder inspection reduces the cost of failure since expensive components have not yet been placed onto the circuit board. Embodiments of the present invention are applicable to a variety of automated optical inspections relative to electronics manufacture. Important inspections include, solder paste inspection, adhesive or glue inspection, post-placement component inspection, lead/ball coplanarity inspection, et cetera.
U.S. Pat. No. 6,750,899, assigned to the Assignee of the present application, provides a height topology inspection system that is able to detect the size and height of solder paste by projecting light through a reticle so as to project a pattern of light onto an area of the printed circuit board. A board transport positions the board to at least two different positions, where each position corresponds to a different phase of the projected light. An image corresponding to each respective position phase is acquired at each position. The relative movement between the sensor and the board is measured, and a processor co-sites the at least two images and constructs a height map image with the co-sited images. The height map image is then used to detect size and height of solder paste on a circuit board.
While U.S. Pat. No. 6,750,899 represents an important contribution to the art of height topology inspection, there is some room for improvement. Accordingly, embodiments of the present invention, as will be described hereinafter, generally improve upon the design provided in U.S. Pat. No. 6,750,899.